The invention relates to a grinding roll for the high-pressure comminution of granular milling material, having a reinforcement with hard bodies which protrude from the surface of the main part of the grinding roll.
According to Schonert, DE 27 08 053 C3, it is known for the comminution of brittle material to press this into so-called flakes by the application of high-pressure load in the roll gap, whereupon the entire material structure breaks and is thereby split into a large number of small fragments. This high-pressure comminution in the roll gap differs from comminution by shearing or rubbing, as happens in a traditional mill, because it is primarily a matter of compressive load. The material which passes through the roll gap hereupon wears the grinding rolls, so that even the grinding rolls used for high-pressure comminution, such as millstones, are subjected to a high level of wear. In order to minimize the wear, it is proposed in DE 100 14 836 A1 to incorporate into the surface of the grinding roll main part hard bodies, which protrude from the main part of the grinding roll. These hard bodies do not here serve primarily as armor for the surface of the main part of the grinding roll, but rather for the structuring of the surface, wherein the material to be comminuted collects on the surface of the main part of the grinding roll, in the spaces between the hard bodies. A layer of the actual material to be comminuted is thereby formed on the surface of the main part of the grinding roll, which layer protects the grinding roll from wearing load. Since in high-pressure comminution it is specifically not a matter of shearing load, but merely of the high pressure to which the material to be comminuted is subjected, the contrarotating grinding rolls run at equal and opposite speed and, where possible, without relative slip, in order to avoid unnecessary and unwanted abrasion of the hard bodies protruding from the surface of the main part of the grinding roll and in order not to continue to erode the layer of material to be comminuted. This type of wear protection is also termed “autogenous wear protection”, because the wear protection layer consists of the actual material which is to be comminuted.
In practice it has been shown that the type of profile on the main part of the grinding wall has an influence on the stability of the autogenous wear protection layer. The tighter the grid of the protruding hard bodies, the more stable, in general, is the autogenous wear protection layer. In practice, a grid which is as tight as possible is therefore chosen in order to stabilize the wear protection layer. This is countered, however, by the fact that, as a result of a high number of hard bodies which also protrude from the wear protection layer, the actual surface of the autogenous wear protection layer is diminished. In addition to the type of grid and the number of hard bodies per unit of area, the shape of the hard bodies is also instrumental for the stability of the formation of an autogenous wear protection layer. Specifically in the start-up of a roller press having grinding rolls which still have no wear protection layer, or in the comminution of materials which are very dry and therefore have little inclination to bake in the spaces between the hard bodies and thereby form a stable layer, the arrangement and shape of the hard bodies on the surface has an influence on the formation of a stable autogenous wear protection layer.
A further aspect in the reinforcement of grinding rolls with hard bodies is the detachability of the hard bodies from the surface of the main part of the grinding rolls. For even with autogenous wear protection, the grinding roll gradually suffers damage through wear and tear. In consequence, the surface of the main part of the grinding rolls is no longer shaped perfectly cylindrically. In a roller press, the gap width and the gap pressure can hence no longer be optimally controlled, whereby the comminuting capacity of the roller press in question is reduced. In consequence thereof, it is necessary to keep supplying the material to the roller gap with greater frequency in a roller press having worn grinding rolls than in a grinding roll having unworn grinding rolls, whereby the number of revolutions of the milling material is increased and hence the grinding capacity of the roller press is reduced.
In order to recondition a grinding roll so as to restore the grinding capacity of the corresponding roller press, the surface of the grinding roll is generally therefore completely replaced. In this context, the hard bodies are removed from the surface, the grinding roll is trued by machining, so that it is again shaped perfectly cylindrically, and the receiving bores for the hard bodies in the surface of the grinding roll are correspondingly deepened to allow the insertion of new or still functional used hard bodies. Although the reconditioning of a grinding roll for high-pressure comminution is very labor-intensive, this type of reconditioning is still economical since the costs of the material of the grinding roll and the hard bodies are very high.
Since the surface of the main part of the grinding roll cannot be trued to a cylindrical format in the presence of the hard bodies, it is necessary to remove the hard bodies beforehand. The removal of all hard bodies from a grinding roll is very laborious, however, because, on the one hand, they are firmly anchored in the main part of the grinding roll and because the hard bodies are additionally driven into the material of the grinding roll by the load applied in the high-pressure comminution, and also because a grinding roll has 50 000 and more individual hard bodies.
In German laid-open application DE 10 2006 010 042 A1, it is proposed, for easier removal of the hard bodies, to provide the hard bodies with a central recess in which a removal tool is intended to engage. In the last-named laid-open application, a rather elongate shape is here proposed for the hard body and, corresponding to the elongate shape, the recess is narrow in relation to the diameter of the hard body. In practice it has been shown that these elongate hard bodies with narrow recess clog up with the material to be comminuted in a form which is difficult to remove again, and also that the recess is too narrow to wedge a powerful and robust removal tool therein. Though the relevant tool size permits removal, under the harsh conditions under which the hard bodies are exchanged a corresponding tool is too sensitive for long-term use.
In order to recondition the grinding roll in an alternative manner, it is proposed in DE 10 2008 014 809 A1 to groove the grinding roll from the side in a lathe and to erode the entire surface with the hard metal bodies beneath the lower end of the hard bodies in the main part of the grinding roll. This method can in principle be implemented in the installed state of the roller press at the site of the roller press itself, yet this type of reconditioning costs an undesirably large quantity of material, whereby the diameter of the grinding roll is also reduced, which ultimately is likewise detrimental to the grinding capacity.